Comparing two and three frequency dB window identification techniques for estimating Antarctic krill density

Three surveys of acoustic data from around South Georgia, collected using a four frequency echosounder (38, 70, 120 and 200 kHz), were used to characterise Antarctic krill. Identification of krill targets was carried out using different permutations of two and three-frequency windows. Density was estimated, from identified krill targets, using the 120 kHz data and an acoustic scattering model. A Bland-Altman analysis with a pre-set nominal cut-off of ≤5 gm^-2 bias between methods identified that only the 120-70 kHz (S_v120-70) identification window was comparable with the regularly used 120-38 kHz (S_v120-38) window, and only the 200-120 kHz and 120-70 kHz (S_v120-70 & S_v200-120) identification window was comparable with the regularly used 120-38 and 200-120 kHz  (S_v120-38 & S_v200-120) three-frequency window.